defiantly need to block sand the primer before laying down paint. I will block with 100-150 grit first then shoot anther coat of primer, a coat may consist of 3-5 wet coats, then block with 220, then 320 then a final wet sanding with 400. you can use a spray can of black paint, spray the panel just lightly enough to give give it a dusting, it will help you see the low areas and get the panel strait.